A wide variety of types of sludge, including municipal sewage sludge, are treated in anaerobic digesters. Historically, anaerobic sludge digestion has been used for stabilizing primary clarifier sludge. More recently, anaerobic digestion has been applied to biological sludge produced by activated sludge and/or trickling filter processes. Anaerobic digestion has also been applied to sludge mixtures including significant (in some cases up to 100%) industrial waste contributions.
Some of these sludges include significant quantities of aerobic microorganisms that are not easily decomposed in an anaerobic digester. Municipal sewage sludge, for example, typically includes significant quantities of manufactured products (ranging from fibrous toiletry products to plastics) or other synthetic products (ranging from long filaments to sheets of bulky materials). In almost all cases, the raw sludge sent to an anaerobic digester is a very diverse and complex mixture of materials ranging from simple inert silt, sand, and soil particles to very complex organic molecules and particles.
During anaerobic digestion, materials are segregated in two directions. Some relatively light materials entrap rising bubbles and are transported to the liquid surface. Similarly, some of the microscopic biomass in raw sludge retains microscopic bubbles and is transported to the surface. Materials having a specific gravity less than the contents of the digester will also rise to the liquid surface through natural buoyancy. In contrast, inert and other heavier materials settle to the bottom of the digester.
Stagnation of material within the anaerobic digester can create process and operational problems. When material stagnates either at the top surface or at the vessel bottom, the digestion process slows substantially, reducing the amount of solids degradation. In addition, rising materials can create a foam, froth, or scum layer that can expand rapidly as new material arrives at the surface and gas expands the surface mass. Accumulating foam, froth, and/or scum at the surface can migrate into gas handling and/or digester safety valve devices, causing problems with the transport of digester gas to other systems.
To avoid such problems, effective digester mixing is important. Current mixing systems are designed to move light material from the digester top to the digester bottom (where it is released and allowed to migrate through the main digester mass back to the top), and to move heavy materials from the digester bottom to the digester top (where they are released and allowed to migrate through the main digester mass back to the bottom).
In general terms, the need for mixing is influenced by a combination of factors. Factors can include, for example, the vessel shape and the general liquid movement throughout the vessel. Experience has shown that egg-shaped vessels, with top and bottom sections conformed to provide adequate slopes for urging the migration of materials toward the center of the vessel, only require volume turnover rates of 3 to 9 times per 24-hour period for sufficient mixing of the digester. The bottom sections of such digesters generally have cone-shaped side slopes of at least 35° measured from the horizontal. The top sections are generally in the shape of an inverted cone or a continuation of a spherical mid-section. These shapes significantly reduce the liquid surface area in the top section from the larger cross-sectional area that is available near the mid-section of the vessel. Egg-shaped vessels (which typically have either a spherical or barrel-shaped section at their mid-section) can utilize the established mid-section shape as a continuation up to the vessel top to reduce the liquid surface area. These various vessel shapes have been widely utilized in Europe and the Far East, and have gained increasing acceptance in North America over the years as they have been installed at a number of sites.
Another factor bearing on the need for mixing is the speed at which sludge migrates to the liquid surface and to the vessel bottom. In large, tall digesters, the migration pathways from the release point back to the top or bottom of the digester can be long, and movement of the materials take a considerable time. Longer migration times can result in stagnation within the digester contents, creating the potential for reduced biological activity and therefore a reduction in the effectiveness of the digester to degrade solids. To prevent stagnation from occurring, more mixing energy per unit volume may be required for larger, taller vessels.
A large majority of the European egg-shaped digesters are mixed with some kind of liquid circulation system. Two general types of systems are used: externally-pumped circulation mixing systems and central draft tube mixing systems.
Externally-pumped circulation mixing systems draw liquid from one section of the vessel and re-inject the liquid at a different location in the vessel. Typically, the mass transport is between the two critical sections of the vessel, the top section and the bottom section. Nozzles are placed at various locations to eliminate the potential for stagnation and prevent short-circuiting within the mixing system for the vessel. In some instances, a nozzle is located above the liquid surface for the control of foam in the upper area of the vessel.
Externally-pumped mixing systems have worked well, but have some operational and economic limitations in large digesters. They can become very complex and expensive as the size of the digester increases. Due to the rising complexities and costs, European engineers generally consider the maximum capacity for a digester utilizing an externally-pumped system to be 3000 to 4000 m3 (approximately 800,000 to 1,000,000 gallons).
Mixing in larger European digesters is typically done with a central draft tube that has a mechanical propeller mixer located near the top of the draft tube. Material is pumped through the draft tube for transport between the top section and bottom section of the digester. The mechanical mixer can have its rotational direction reversed so that the direction of flow through the draft tube can be either up or down. For foam control, the mixer shaft is usually furnished with a splash disk located above the operating liquid level, although the splash disk has not proven to be an effective method of foam control in many cases. Nonetheless, many North American facilities have applied the well-established European mechanical mixer with central draft tube design with good success.
Several American and Canadian facilities have recently utilized a patented system that couples one or more jet pumps to a central draft tube. The jet pumps are located at the bottom and at the top of the central draft tube and use the momentum of a high-velocity discharge directed at the mouth of a venturi section on the end of the draft tube to induce the surrounding liquid into the mouth of the draft tube, thus providing a strong pumping action. In jet pump systems, additional high-velocity discharge nozzles have been positioned above the liquid surface to provide a more effective method of foam, froth, and scum control. For digesters up to about 4,900 m3 (1,300,000 gallons), jet pump draft tube mixing systems have provided a simpler and more economical approach to mixing than the systems developed in Europe.
It appears to some in the industry that the pumping requirements for moving large quantities of sludge between the top and bottom sections of tall digesters effectively limits the size of digesters using a single, central draft tube mixing arrangement to about 5,700 m3 (1,500,000 gallons), especially with respect to a jet pump system. One reason for this is that larger digesters appear to require more mixing energy, on a per-unit volume basis, than smaller digesters. Another is the heterogeneous nature of wastewater sludge. Various components of sludge have different specific gravities, which cause solids containing different proportions of those components to migrate to different sections in the digester water column. A draft tube mixing system is designed to develop a strong flow of liquid with a relatively low energy. When the system is required to move a water column in the draft tube that is loaded with bottom material (which is significantly heavier than the water alongside the draft tube), the efficiency of the system decreases. As the height of the draft tube increases, the mixing capacity of a conventional draft tube system decreases, increasing the potential for stagnation within the vessel.
The ability to control foam, froth, and/or scum can also be affected by the size of the digester. As the volume of an egg-shaped and/or cone-cylinder-cone digester increases, the ratio of the liquid surface area (at the operating liquid level in the digester) to the digester volume decreases. For example, the liquid surface area through which gas must pass to enter the cylindrical gas collection dome of a large, egg-shaped digester with a volume of 10,000 m3 is only roughly twice that of a small, egg-shaped digester with a volume of 2,000 m3. As a result, the ratio of the surface area to digester volume of the larger digester is about 40% of the ratio for the smaller digester. The larger digester handles five times the sludge volume and therefore produces five times the gas of the smaller digester. Thus, the larger digester is passing five times the gas through a surface area that is only about twice as large as the surface area in the smaller digester. The smaller ratio of surface area to digester volume reduces the capacity of the larger digester to control foam, froth, and/or scum in comparison to the smaller digester. Viscosity and/or unique sludge molecular properties can further diminish the foam, froth, and/or scum handling capacity of large egg-shaped digesters compared to the corresponding capacity in small egg-shaped digesters.
Operational issues may also arise in connection with wastewater facilities that divide the microbiological reactions of the digestion process into two or more vessels operated in series. These systems vary in configuration and process operation, but generally divide the digestion process into two phases referred to as an acid phase and a gas phase.
A relatively constant hydraulic retention time is usually desired during the acid phase in order to achieve optimum process results. If the population of the community served by a wastewater treatment system stayed static, and the waste sludge flowed under steady-state, homogeneous conditions, then the selection and sizing of the acid phase digester could be relatively easy and straightforward. However, feed rates (and water quality) typically change in municipal wastewater facilities, so operational control of the digester facility retention times is generally needed, especially in the acid phase of the two-phase digestion process.
To accommodate changing flow rates in the municipal wastewater facility, acid phase digester vessel(s) are preferably equipped to handle volume increases and/or reductions depending upon the incoming sludge quantity and quality. Typically, retention time is adjusted by varying the liquid depth in the vessel. These depth variations can be significant. In some cases, the liquid level may vary as much as 40%.
Egg-shaped digesters have typically been operated at a relatively-constant liquid level. Under full-depth conditions, the liquid surface area is small and foam spray knockdown systems can be effective at breaking up emulsions that can otherwise develop into foam, froth, and/or scum. Once the emulsions are broken up, the solids and water can be captured by the draft tube and transported from the surface to the bottom section of the vessel.
However, when liquid depth within the vessel is reduced, the liquid surface area can grow by several times. The larger area allows the light solids to escape capture either in a draft tube or through an overflow weir or inlet associated with the discharge system. A significant drop in the operating liquid level can also significantly reduce the effectiveness of the central draft tube mixing system, especially when the level drops below the top of the draft tube. This phenomenon makes mixing in large egg-shaped digesters much more difficult.
The mixing arrangements conventionally used to deal with liquid depth variations in egg-shaped or acid-phase digesters are complex. Typically, the mixing system relies on an externally-pumped circulation system that includes a series of pump suction and discharge nozzles designed to allow liquid to be withdrawn from any of a variety of different liquid depths in the vessel and returned to one of many different depth options for forced circulation. The locations of the nozzles are positioned throughout the vessel to impart mixing energy in all areas of the vessel (including the top and bottom sections of the vessel) without creating short-circuiting in the system. Coordination of these various nozzles makes the system complex, and thus expensive to construct and operate.